Monoclonal antibodies directed against protein antigens are used as probes to study antibody-protein interactions and to study developmentally regulated antigens in normal and neoplastic development. In order to define the complementary structure of an antibody and a protein epitope as precisely as possible, antigenic regions and specific epitopes recognized by monoclonal antibodies to avian lysozyme are mapped by comparing antibody reactivity with related lysozymes, and the antibodies are analyzed structurally by sequencing, crystallography and computer modelling. Structurally and functionally related antibodies are compared to determine genetic mechanisms underlying anti-proteins specificity. Experiments are in progress to generate monoclonal antibodies to specific tumor associated antigens, and to onc gene protein products; these antibodies will be used to study these proteins in normal development and in plasmacytoma cells.